Photoelectric conversion apparatus

ABSTRACT

The invention relates to a technique removing a dark voltage noise signal occurring at the time of light shielding of a photoelectric conversion device, simplifying the system configuration such that the cost and occupying area is reduced, and suppressing variation among a plurality of buffer amplifiers. In order to obtain such a photoelectric conversion device, a photoelectric conversion element  1  includes: a first accumulating unit  6  that accumulates an output signal outcoming from the photoelectric conversion element during a first period; a second accumulating unit  7  that accumulates an output signal outcoming from the photoelectric conversion element during a second period different from the first period; first and second switch units  4  and  5  used for selectively introducing the output signal from the photoelectric conversion element  1  to the first or second accumulating unit; and an output unit that outputs output signals from the first and second accumulating units  6  and  7  by inversing the polarity of the first accumulating unit  6.

TECHNICAL FIELD

The present invention relates to a photoelectric conversion device thatis used for a facsimile, a copier, a digital camera, etc., and canremove dark voltage noise occurring at the time of shielding light.

BACKGROUND ART

In general, a CCD is mainly used as an image sensor for a facsimile, acopier, a digital camera, etc., but in recent years, an amplificationtype photoelectric conversion device that includes an amplifying elementsuch as a MOS transistor or a bipolar transistor in each pixel unit hasbeen used. In an amplification type photoelectric conversion device, itis important to remove noise in order to read a signal with highsensitivity.

FIG. 6 shows an example of an amplification type photoelectricconversion device according to the related art. In a photoelectricconversion element 101, even though light is not incident thereon,current flows, which generates noises. Actually, in a case of reading asignal generated by light that is incident onto the photoelectricconversion device 101, it is necessary to remove dark voltage noise andnoise caused due to variation of a dark voltage (hereinafter, referredto as a noise signal component). First, in order to output a noisesignal component in a case in which light is not incident onto thephotoelectric conversion element 101, a selecting unit, which iscomposed of a MOS transistor 104, is turned on through a bufferamplifier 103, and a noise signal component, such as electric charge, isaccumulated in a noise accumulating unit 106. After elapse of arbitraryaccumulation time, the MOS transistor 104 is turned off. Next, thesignal generated by the incident light onto the photoelectric conversionelement 101 passes through the buffer amplifier 103 so as to turn off aMOS transistor 105, thereby charge is accumulated in a signalaccumulating unit 107. After an elapse of arbitrary accumulation time,the MOS transistor 105 is turned off.

At this time, electric charge owing to dark voltage noise and electriccharge when light is incident are accumulated in the accumulating units106 and 107, respectively. Then, MOS transistors 110 and 111 aresimultaneously turned on and thus the signal components are input to adifferential amplifier 114 through buffer amplifiers 112 and 113,respectively. The differential amplifier 114 obtains the differencebetween the signal components from the accumulating units 107 and 106,and obtains the signal occurring due to a real incident light byremoving the noise signal component of the accumulating unit 106.Finally, MOS transistors 108 and 109 are turned on such that charge inthe accumulating units 106 and 107 is reset. Further, in order torefresh the remaining charge of the photoelectric conversion element101, a MOS transistor 102 is turned on such that an offset voltage isapplied. In this way, it is possible to remove the noise signalcomponent and to exactly obtain a net signal due to real incident light(refer to, for example, JP-A-9-205588 and JP-A-8-255027).

However, the differential amplifier 114 requires various elements (inparticular, at least, eight MOS transistors, one capacitor, and oneresistor) as shown in FIG. 5. Therefore, there is a problem in that thearea occupied by the differential amplifier does not reduce as the sizeof the photoelectric conversion device reduces. Further, two bufferamplifiers, that is, the buffer amplifier 112 for dark voltage noise andthe buffer amplifier 113 for an optical signal are required. This is aproblem in terms of the occupied area. Similar to the differentialamplifier, the buffer amplifiers 112 and 113 also generally include aplurality of MOS transistors. Further, in order to make the bufferamplifiers or the differential amplifier function generally with highaccuracy, there are cases of using even more elements, whereby it isdifficult to disregard the area problem.

Furthermore, variation of an output voltage occurs between twoamplifiers, that is, the buffer amplifier 112 for dark voltage noise andthe buffer amplifier 113 for an optical signal. Therefore, actually,there is a possibility that the dark voltage noise signal component isnot completely removed and thus an exact signal is not read. Inparticular, the buffer amplifiers 112 and 113 are amplifiers having afeedback amplification factor of 1 and include a plurality of MOStransistors or bipolar transistors. Therefore, property error anddeviation of a transistor element due to a manufacturing process of asemiconductor integrated circuit or the like may occur and inparticular, the possibility that variation of an output voltage occurstends to increase as the number of elements increases.

The present invention has been finalized in view of the above-mentioneddrawbacks in the related art, and it is an object of the invention toprovide a photoelectric conversion device that removes a noise signalcomponent and reads a signal due to incident light, and that reduces thecost and occupied area and removes variation among a plurality of bufferamplifiers by simplifying the system configuration.

DISCLOSURE OF THE INVENTION

The invention claimed in claim 1 provides a photoelectric conversiondevice including: a photoelectric conversion element; a firstaccumulating unit that accumulates an output signal outcoming from thephotoelectric conversion element during a first period; a secondaccumulating unit that accumulates an output signal outcoming from thephotoelectric conversion element during a second period different fromthe first period; first and second switch units used for selectivelyintroducing the output signal from the photoelectric conversion elementto the first or second accumulating unit; and an output unit thatoutputs the output signals from the photoelectric conversion elementaccumulated in the first and second accumulating units at the same timeby inversing the polarity of the first accumulating unit.

The invention claimed in claim 2 provides the photoelectric conversiondevice claimed in claim 1, in which the output unit includes: a thirdswitch unit selectively applying a reference voltage to the connectionpoint between the first witch unit and the first accumulating unit; afourth switch unit selectively applying a reference voltage to the firstand second accumulating units; and a fifth switch unit that is connectedto the connection point between the second accumulating unit and thesecond switch unit and reads signals from the first and secondaccumulating units.

The invention claimed in claim 3 provides a photoelectric conversiondevice including: a photoelectric conversion element; a firstaccumulating unit that accumulates an output signal outcoming from thephotoelectric conversion element during a first period; a secondaccumulating unit that accumulates an output signal outcoming from thephotoelectric conversion element during a second period different fromthe first period; first and second switch units used for selectivelyintroducing the output signal from the photoelectric conversion elementto the first or second accumulating unit; a third switch unitselectively applying a reference voltage to the connection point betweenthe first switch unit and the first accumulating unit; a fourth switchunit selectively applying a reference voltage to the first and secondaccumulating units; and a fifth switch unit that is connected to theconnection point between the second accumulating unit and the secondswitch unit and reads signals from the first and second accumulatingunits.

The invention claimed in claim 4 provides a photoelectric conversiondevice including: a photoelectric conversion element; a firstaccumulating unit that accumulates an output signal outcoming from thephotoelectric conversion element during a first period; a secondaccumulating unit that accumulates an output signal outcoming from thephotoelectric conversion element during a second period different fromthe first period; first and second switch units used for selectivelyintroducing the output signal from the photoelectric conversion elementto the first or second accumulating unit; a third switch unitselectively applying a reference voltage to the first accumulating unitwhen reading; a fourth switch unit selectively applying a referencevoltage to the first and second accumulating units when the outputsignal from the photoelectric conversion element is accumulated; and afifth switch unit that outputs output signals from the first and secondaccumulating units at the same time at the time of reading.

The invention claimed in claim 5 provides a photoelectric conversiondevice which includes: a plurality of photoelectric conversion elements,each forming a pixel, arranged in a matrix; first accumulating unitsthat accumulate output signals outcoming from the photoelectricconversion elements during a first period; second accumulating unitsthat accumulate output signals outcoming from the photoelectricconversion elements during a second period different from the firstperiod; first and second switch units used for selectively introducingthe output signals from the photoelectric conversion elements to thefirst or second accumulating units; third switch units selectivelyapplying a reference voltage to the first accumulating units whenreading; fourth switch units selectively applying a reference voltage tothe first and second accumulating units when the signals from thephotoelectric conversion elements are accumulated; and fifth switchunits that output signals from the first and second accumulating unitsat the same time at the time of reading, and which removes noise foreach pixel.

The invention claimed in claim 6 provides the photoelectric conversiondevice claimed in any one of claims 1 to 5, in which each of the switchunits is composed of a MOS transistor.

The invention claimed in claim 7 provides the photoelectric conversiondevice claimed in any one of claims 1 to 5, in which the fourth switchunit includes: a first MOS transistor selectively applying a referencevoltage to the first accumulating unit; a second MOS transistorselectively applying a reference voltage to the second accumulatingunit; and a third MOS transistor that connects the connection pointbetween the first accumulating unit and the first MOS transistor to theconnection point between the second accumulating unit and the second MOStransistor.

(Effect) According to the invention, an output circuit is configured toinverse the polarity of the first accumulating unit accumulating thenoise signal component, derives an output signal from the differencebetween the first accumulating unit and the second accumulating unitaccumulating the optical signal component according to the incidentlight, and outputs the output signal to an output line. Therefore, it isunnecessary to configure a circuit such as a differential amplifier andso on, and thus the system configuration is simplified, thereby capableof reducing the number of elements and the layout area. Further, it ispossible to remove the noise signal component in each pixel unit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram illustrating a photoelectric conversiondevice according to a first embodiment of the invention.

FIG. 2 is a timing chart illustrating the photoelectric conversiondevice according to the first embodiment of the invention.

FIG. 3 is a circuit configuration diagram showing a photoelectricconversion device according to the first embodiment of the invention inwhich a plurality of pixels are two-dimensionally arranged.

FIG. 4 is a circuit diagram illustrating a photoelectric conversiondevice according to a second embodiment of the invention.

FIG. 5 is a circuit diagram showing the basic configuration of adifferential amplifier.

FIG. 6 is a circuit diagram illustrating a photoelectric conversiondevice according to the related art.

REFERENCE NUMERALS

1 PHOTOELECTRIC CONVERSION ELEMENT

3 BUFFER AMPLIFIER

4 FIRST SWITCH UNIT

5 SECOND SWITCH UNIT

6 FIRST ACCUMULATING UNIT

7 SECOND ACCUMULATING UNIT

8 THIRD SWITCH UNIT

9 FOURTH SWITCH UNIT

9 a FIRST MOS TRANSISTOR

9 b SECOND MOS TRANSISTOR

9 c THIRD MOS TRANSISTOR

10 FIFTH SWITCH UNIT

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is a circuit diagram illustrating a first embodiment of theinvention. A photoelectric conversion element 1 composed of a photodiodeis provided in each pixel unit. The photoelectric conversion element 1is connected to a buffer amplifier 3. The buffer amplifier 3 isconnected to the sources of first and second switch units 4 and 5 eachcomposed of a MOS transistor. The drain of the first switch unit 4composed of a MOS transistor is connected to a first accumulating unit 6composed of a capacitor or the like, and the first accumulating unit 6accumulates noise signals from the photoelectric conversion element 1.The connection point between the first switch unit 4 composed of a MOStransistor and the first accumulating unit is grounded through a thirdswitch unit 8 composed of a MOS transistor. Further, the drain of thesecond switch unit 5 composed of a MOS transistor is connected to asecond accumulating unit 7 composed of a capacitor or the like, and thesecond accumulating unit 7 accumulates noise signals from thephotoelectric conversion element 1. Each switch unit is not limited to aMOS transistor but may be composed of a bipolar transistor.

A source of a fifth switch unit 10 composed of a MOS transistor isconnected to the connection point between the second switch unit 5 andthe second accumulating unit 7, and a drain of the fifth switch unit 10is connected to the buffer amplifier 11. A source of a fourth switchunit 9 composed of a MOS transistor is connected to the connection pointbetween the first accumulating unit 6 and the second accumulating unit7, and a drain of the fourth switch unit 9 is grounded. A bias voltageis applied to the photoelectric conversion element 1 through a switchunit 2 composed of a MOS transistor.

FIG. 2 is a timing chart illustrating the first embodiment of theinvention and illustrates the operation and the configuration of thefirst embodiment.

First, when driving pulses WTL and WTG to be applied to the gates of theMOS transistors are turned on (Step 1), light is incident onto thephotoelectric conversion element 1 so as to generate an optical signaland the optical signal is accumulated in the second accumulating unit 7within an arbitrary period (Step 2). Subsequently, when the drivingpulse WTL is turned off and a driving pulse VB is turned on, a biasvoltage is applied to refresh the photoelectric conversion element 1(Step 3). Directly after the refresh operation, when a driving pulse WTDis turned on, a noise signal is accumulated in the first accumulatingunit 6 within an arbitrary period (Step 4). At this time, the first andsecond accumulating units 6 and 7 have the accumulated noise signal andthe accumulated optical signal, respectively. The application timeperiod of the driving pulse WTD applied to the MOS transistor 4 (noisesignal accumulating period) and the application time period of thedriving pulse WTL applied to the MOS transistor 5 (optical signalaccumulating period) are preferably set to be almost the same in orderto precisely obtain a net signal by removing a noise signal componentfrom an optical signal component.

Then, when the driving pulse WTG is turned off (Step 5) and a drivingpulse RDG is turned on (Step 6), the polarity of the first accumulatingunit 6 is inversed. When the varied polarity is added to the polarity ofthe second accumulating unit 7, the voltage corresponding to the noisesignal component accumulated in the first accumulating unit 6 issubtracted from the voltage corresponding to the optical signalcomponent accumulated in the second accumulating unit 7, therebyremoving the noise signal component. Further, when a driving pulse RD isturned on, a net optical signal without noise signal component isobtained (Step 7).

It is assumed that the potential value of the noise signal component,which is accumulated in the first accumulating unit 6 at step 2, inrespect to a reference voltage is +Vd and the potential value of theoptical signal component, which is accumulated in the secondaccumulating unit at step 4, is +VL. The accumulating units 6 and 7 aredirectly connected to each other after steps 5 and 6. Therefore, thepolarity of the accumulating unit 6 becomes −VD and the potential of theconnection point between the second switch unit and the accumulatingunit 7 becomes (VL−VD). Next, at step 7, when the driving pulse RD isturned on, the voltage (VL−VD) is applied to the buffer amplifier 11.Therefore, the noise signal component is removed from the optical signalcomponent and thus a net signal is output.

FIG. 3 shows a two-dimensional basic pixel structure of photoelectricconversion device. The fifth switch units 10, each composed of a MOStransistor, are sequentially turned on by a scanning circuit 13 such asa shift register such that signals are outcoming from individual pixelsby the driving pulse RD. The output signal sequentially output is sentto the buffer amplifier 11 through a common output line 12.

FIG. 4 is a circuit diagram illustrating a second embodiment of theinvention. The fourth switch unit is composed of three MOS transistorsunlike that in the first embodiment. A first MOS transistor 9 aselectively applies the reference voltage to the first accumulating unit6, and a second MOS transistor selectively applies the reference voltageto the second accumulating unit 7. In regards to a third MOS transistor,the source of the third MOS transistor 9 c is connected to theconnection point between the first accumulating unit 6 and the secondMOS transistor 9 b and the drain of the third MOS transistor 9 c isconnected to the connection point between the second accumulating unit 7and the second MOS transistor.

The operation in the second embodiment will be described (the timingchart is not shown). Basically, the operation follows the timing chartof FIG. 2 in the first embodiment. However, when a driving pulse WGT1 isturned on, a noise signal component is accumulated in the firstaccumulating unit 6, and when a driving pulse WGT2 is turned on, anoptical signal component is accumulated in the second accumulating unit7. Further, when the driving pulses WGT1 and WGT2 are turned off, thedriving pulse RDG is turned on, and then a driving pulse WGT3 is turnedon, the noise signal component of the first accumulating unit 6 isremoved.

At this time, the photoelectric conversion element 1 is composed of aphotodiode 1 but may be an element formed by a combination of acapacitor and a bipolar transistor. In this case, charge, which isgenerated by light that is incident onto the base of the bipolartransistor, is accumulated and current between the emitter electrode andthe collector electrode is controlled by the charge amount. Further,when a pulse is applied to the capacitor electrode, a refresh operationis performed.

The reference voltage in the first and second embodiment is set to aground voltage but may be set to a variable voltage (not shown). This isavailable in a case in which parasitic capacitance, generated when thefirst and second switch units 4 and 5 are transistors, can bedisregarded. The first accumulating unit 6 has noise which is an originof the parasitic capacitance as well as a dark voltage noise signalcomponent, and the second accumulating unit 7 has noise which is anorigin of the parasitic capacitance as well as the optical signalcomponent. Therefore, it is preferable to set the variable voltage to anarbitrary value so as to remove the voltage which is the origin of theparasitic capacitance. Further, a circuit for removing the noise that isthe origin of the parasitic capacitance may be configured.

In the description of the invention, the optical signal component isfirst accumulated but the noise signal component may be firstaccumulated so as to remove the noise signal component. Further, aconfiguration in which the first accumulating unit 6 is used foraccumulating the optical signal component, and the second accumulatingunit 7 is used for accumulating the noise signal component can berealized by only inverting the positive and the negative of thepotential.

The invention has been described in detail in reference to the aboveembodiments but it will be understood by those skilled in the art thatvarious modifications and changes can be made without departing from thespirit and scope of the invention.

This application is based on Japanese Patent Application No. 2003-307706filed in the Japanese Patent Office on 29th day of Aug. 2003 and refersto the context of Japanese Patent Application No. 2003-307706 herein.

INDUSTRIAL APPLICABILITY

According to the invention, since it is unnecessary to configure acircuit such as a differential amplifier or an extra buffer amplifier,it is possible to simplify the system configuration and to reduce thenumber of elements and the layout area. Further, since two amplifiers,that is, a buffer amplifier for dark voltage noise and a bufferamplifier for an optical signal are not required, it is possible tofurther reduce the area. Furthermore, since a problem does not occurwhen the dark voltage noise component is not effectively removed due tooccurrence of variation in the output voltage, it is possible to read anexact signal.

1. A photoelectric conversion device comprising: a photoelectricconversion element; a first accumulating unit that accumulates an outputsignal outcoming from the photoelectric conversion element during afirst period; a second accumulating unit that accumulates an outputsignal outcoming from the photoelectric conversion element during asecond period different from the first period; first and second switchunits used for selectively introducing the output signal from thephotoelectric conversion element to the first or second accumulatingunit; and an output unit that outputs the output signals from thephotoelectric conversion element accumulated in the first and secondaccumulating units at the same time by inversing the polarity of thefirst accumulating unit.
 2. The photoelectric conversion deviceaccording to claim 1, wherein the output unit includes: a third switchunit selectively applying a reference voltage to the connection pointbetween the first switch unit and the first accumulating unit; a fourthswitch unit selectively applying a reference voltage to the first andsecond accumulating units; and a fifth switch unit that is connected tothe connection point between the second accumulating unit and the secondswitch unit so as to read out signals from the first and secondaccumulating units.
 3. A photoelectric conversion device comprising: aphotoelectric conversion element; a first accumulating unit thataccumulates an output signal outcoming from the photoelectric conversionelement during a first period; a second accumulating unit thataccumulates an output signal outcoming from the photoelectric conversionelement during a second period different from the first period; firstand second switch units used for selectively introducing the outputsignal from the photoelectric conversion element to the first or secondaccumulating unit; a third switch unit selectively applying a referencevoltage to the connection point between the first switch unit and thefirst accumulating unit; a fourth switch unit selectively applying areference voltage to the first and second accumulating units; and afifth switch unit that is connected to the connection point between thesecond accumulating unit and the second switch unit so as to read outsignals from the first and second accumulating units.
 4. A photoelectricconversion device comprising: a photoelectric conversion element; afirst accumulating unit that accumulates an output signal outcoming fromthe photoelectric conversion element during a first period; a secondaccumulating unit that accumulates an output signal outcoming from thephotoelectric conversion element during a second period different fromthe first period; first and second switch units used for selectivelyintroducing the output signal from the photoelectric conversion elementto the first or second accumulating unit; a third switch unitselectively applying a reference voltage to the first accumulating unitwhen reading; a fourth switch unit selectively applying a referencevoltage to the first and second accumulating units when the outputsignal from the photoelectric conversion element is accumulated; and afifth switch unit that outputs output signals from the first and secondaccumulating units at the same time at the time of reading.
 5. Aphotoelectric conversion device comprising: a plurality of photoelectricconversion elements, each forming a pixel, arranged in a matrix; firstaccumulating units that accumulate output signals outcoming from thephotoelectric conversion elements during a first period; secondaccumulating units that accumulate output signals outcoming from thephotoelectric conversion elements during a second period different fromthe first period; first and second switch units used for selectivelyintroducing the output signals from the photoelectric conversionelements to the first or second accumulating units; third switch unitsselectively applying a reference voltage to the first accumulating unitswhen reading; fourth switch units selectively applying a referencevoltage to the first and second accumulating units when the signals fromthe photoelectric conversion elements are accumulated; and fifth switchunits that output signals from the first and second accumulating unitsat the same time at the time of reading, wherein noise is removed foreach pixel.
 6. The photoelectric conversion device according to any oneof claims 1 to 5, wherein each of the switch units is composed of a MOStransistor.
 7. The photoelectric conversion device according to any oneof claims 1 to 5, wherein the fourth switch unit includes: a first MOStransistor selectively applying a reference voltage to the firstaccumulating unit; a second MOS transistor selectively applying areference voltage to the second accumulating unit; and a third MOStransistor that connects the connection point between the firstaccumulating unit and the first MOS transistor to the connection pointbetween the second accumulating unit and the second MOS transistor.